Ferromagnetic interactions in hosted bipartite materials—generalized-double-exchange andgeneralized-superexchange interactions

Abstract

Defect-induced magnetism in dilute magnetic semiconductors challenges our understandingof magnetism in solids. Theories based on conventional superexchange or double-exchangeinteractions cannot explain long range magnetic order at concentrations below thepercolation threshold in these materials. On the other hand, the codoping-inducedmagnetism, which can explain magnetic interactions below the percolation threshold, haseluded explanation. In this work we propose that defect-induced magnetism in codopednon-magnetic materials can be viewed within a molecular generalization of the atomicdouble-exchange and superexchange interactions applied to an arbitrary bipartite latticehosted by (or embedded in) defect-free non-magnetic materials. In this view, the crucialfactor for the development of magnetism appears to be the defect complementarity of thecodopants. We demonstrate this by taking ZnO and GaN (the most widely studied dopedoxide and nitride magnetic semiconductors, respectively) as host materials and performtheoretical calculations using ab initio methods after codoping them with transition metalimpurities for a variety of configurations. Our results indicate that the magneticcoupling among the induced and/or doped magnetic moments takes the form of aninteraction among spin-polarized molecular units which is facilitated by the formationof the hosted bipartite codopant structures. The universality of the proposedmechanism is further supported by earlier results referring to the rhombohedralC60-based polymers.